Filter: Add route attribute gw_mpls_stack

Add route attribute gw_mpls_stack to make MPLS stack of route nexthop
accessible from filters. Its type is T_CLIST, which is really not correct
(as it is a list, while T_CLIST is a set). Therefore, we keep this
attribute *undocumented* and it will be *changed* without further notice.

Based on a patch from Trisha Biswas <tbiswas@fastly.com>, thanks!

lib/ip.h

@@ -392,6 +392,7 @@ static inline ip6_addr ip6_ntoh(ip6_addr a)
 #define MPLS_MAX_LABEL 0x100000
 
 #define MPLS_MAX_LABEL_STACK 8
+#define MPLS_MAX_LABEL_STRING MPLS_MAX_LABEL_STACK*12 + 5
 typedef struct mpls_label_stack {
   uint len;
   u32 stack[MPLS_MAX_LABEL_STACK];